Browse to the board where you wish to post and click the New Topic button (positioned by default at both the top and the bottom of the board, as shown in the User Help Introduction), which should take you to the Start new topic screen. While this presents a number of options, the two most important are the Subject field and main Message field (not labeled as such, but obvious by its size). So enter your subject and start to type (or paste) your message into the main text area, noting that a message missing either a subject or a body will not be accepted for posting until it has both. If you're happy with your plain text message, you can then post it by clicking the Post button and/or preview it first by clicking the Preview button, but there are many further options available to make things that little bit more interesting.
